patch 8.1.1446: popup window callback not implemented yet

Problem:    Popup window callback not implemented yet.
Solution:   Implement the callback.
diff --git a/src/testdir/test_popupwin.vim b/src/testdir/test_popupwin.vim
index 6a2ef24..6e05919 100644
--- a/src/testdir/test_popupwin.vim
+++ b/src/testdir/test_popupwin.vim
@@ -586,3 +586,13 @@
   delfunc MyPopupFilter
   popupclear
 endfunc
+
+func Test_popup_close_callback()
+  func PopupDone(id, result)
+    let g:result = a:result
+  endfunc
+  let winid = popup_create('something', {'callback': 'PopupDone'})
+  redraw
+  call popup_close(winid, 'done')
+  call assert_equal('done', g:result)
+endfunc